Question: 9 pencils cost $15.21. Which equation would help determine the cost of 11 pencils?
Answer: There are several equations that could help determine the cost, each with a slightly different approach. We know the cost of 9 pencils. We want to know the cost of 11 pencils. We can write the numbers of pencils as a proportion: $\dfrac{9}{11}$ We know 9 pencils costs $15.21. We can let $x$ represent the unknown cost of 11 pencils. The proportion of these costs can be expressed as: $\dfrac{\$15.21}{x}$ The cost changes along with the number of pencils purchased, and so the two proportions are equivalent. $\dfrac{9}{11} = \dfrac{\$15.21}{x}$